1. Field of the Invention
The present invention relates to a control apparatus for a four wheel drive vehicle having a differential limiting mechanism on at least either one of a front wheel or a rear wheel side and in which a transfer clutch freely engageable and disengageable is provided between the front and rear wheel sides for changing driving wheels.
2. Related Art
In general, it is known that a tight corner braking phenomenon easily happens to be generated by a differential limiting torque in implementing a differential limiting control according to a difference in wheel speeds between left and right wheels unless a target differential speed between the left and right wheels is set in consideration of the difference in a trace of the respective wheels when steering.
On the other hand, for example, JP-A-2001-71776 discloses the differential limiting of a center differential according to the front and rear wheel differential.
In adopting the differential limiting control of the center differential like the aforementioned related art on a vehicle equipped with a mechanical differential limiting mechanism on either one of front or rear wheel side for the purpose of sporty running, a mechanical differential limiting control between the left and right wheels interferes with the differential limiting control of the center differential. Then an unnecessary control is implemented at the center differential, whereby there may occur a risk that the mechanical differential limiting mechanism does not sufficiently actuate or an unexpected braking phenomenon is generated.
Namely, in a condition where a slippage of the inside wheel while turning is restrained by the mechanical differential limiting control between the left and right wheels, where by the speeds of the turning inner and outside wheels become identical. An excessive driving torque of the inside wheel is shifted to the outside wheel, whereby there is generated a yaw moment at a front portion of the vehicle toward a turning direction, which is desirable for a sporty running. However, in this case, this condition is the regarded as the slippage of the inside wheel and the differential limiting control of the center differential, the torque distribution to the front and rear axles changes vehicle behaviors, this leading to the risk that the tight corner braking phenomenon is generated.
The present invention was made in view of the situations, and an object thereof is to provide a control apparatus for a four wheel drive vehicle having a differential limiting mechanism on at least either one of a front wheel side or a rear wheel side which can not only prevent unnecessary spins of a inside wheel while turning by the differential limiting mechanism between left and right wheels when a sporty running is being performed but also avoid a tight corner braking phenomenon which would occur when the vehicle is steered largely at a low speed.
With a view to attaining the object, according to a first aspect of the present invention, there is provided a control apparatus for a four wheel drive vehicle having differential limiting unit on at least either one of a front wheel or the rear wheel side, the control apparatus having the turning condition determining unit for determining the turning condition of a vehicle, a left and right inside wheel actual differential speed calculating unit for calculating an actual left and right wheel differential speed according to a turning state of the vehicle, a target differential speed setting unit for setting a target left and right wheel differential speed at the differential limiting unit, a differential limiting torque calculating unit for setting a differential limiting torque based on an information at the differential limiting unit at 0, an inside wheel speed falls below an outside wheel speed by a preset threshold value in the turning state and for calculating a differential limiting torque based on the information at the differential limiting unit based on the target left and right wheel differential speed and the actual left and right wheel differential speed when the inside wheel speed exceeds an outside wheel speed by the preset threshold value in the turning state, and a front and rear wheel differential limiting torque setting unit for setting a front and rear wheel differential limiting torque based on a differential limiting torque based on the information which is calculated at the differential limiting torque calculating unit.
In addition, according to a second aspect of the present invention, there is provided a control apparatus for the four wheel drive vehicle as set forth in the first aspect of the present invention, wherein the threshold value used at the differential limiting torque calculating unit to compare the inside wheel speed with the outside wheel speed at 0 of the turning state.
Furthermore, according to a third aspect of the present invention, there is provided the control apparatus for the four wheel drive vehicle as set forth in the first or second aspect of the present invention, wherein in the event that the turning state determining unit determines the turning state of the vehicle substantially as a straight running condition, the actual actual left and right wheel differential speed calculating unit regards an actual left and right wheel differential speed at the differential limiting unit as an absolute value of a left and right wheel differential speed. The differential limiting torque calculating unit calculates a differential limiting torque based on the information at the differential limiting unit and the absolute value of the left and right wheel differential speed.
In addition, according to a fourth aspect of the present invention, there is provided the control apparatus for the four wheel drive vehicle as set forth in any of the first to third aspects of the present invention, wherein the turning state determining unit determines a turning state of the vehicle from at least one of a lateral acceleration, a yaw rate or a steering angle.
Furthermore, according to a fifth aspect of the present invention, there is provided the control apparatus for the four wheel drive vehicle as set forth in the fourth aspect of the present invention, wherein the turning state determining unit determines that the vehicle is in substantially a straight running state when an absolute value of at least one of the lateral acceleration, the yaw rate or the steering angle is smaller than a predetermined value.
According to a sixth aspect of the present invention, there is provided the control method for the four wheel drive vehicle having the differential limiting unit on at least either one of the front wheel or the rear wheel side, the control method having steps of: determining the turning state of the vehicle; calculating the actual left and right wheel differential speed at the differential limiting unit according to the turning state of the vehicle; setting the target left and right wheel differential speed at the differential limiting unit; setting a differential limiting torque based on the information at 0 of the differential limiting unit, in the event that at the differential limiting unit, an inside wheel speed falls below that of the outside wheel speed by the preset threshold value in the turning state; calculating the differential limiting torque based on the information at the differential limiting unit based on the target left and right wheel differential speed and the actual left and right wheel differential speed, in the event that the inside wheel speed exceeds the outside wheel speed by the preset threshold value in the turning state; and setting the front and rear wheel differential limiting torque based on the differential limiting torque based on the information at the differential limiting unit which is calculated at the previous step.
According to a seventh aspect of the present invention, there is provided the control method as set forth in the sixth aspect, wherein the threshold value used to compare the inside wheel speed with the outside wheel speed in the turning state 0.
According to an eighth aspect of the present invention, there is provided the control method as set forth in the sixth or seventh aspect, wherein in the event that the turning state is determined the turning state of the vehicle substantially as a running straight condition, the actual left and right wheel differential speed calculating step regards the actual left and right wheel differential speed at the differential limiting unit as an absolute value of the left and right wheel differential speed, and the differential limiting torque calculating step calculates the differential limiting torque based on the information at the differential limiting unit based on the target left and right wheel differential speed and the absolute value of the left and right wheel differential speed.
According to a ninth aspect of the present invention, there is provided the control method as set forth in any one of the sixth to eighth aspects, wherein the turning state is determined from at least one of the lateral acceleration, the yaw rate or the steering angle.
According to a tenth aspect of the present invention, there is provided the control method as set forth in the ninth aspect, wherein the turning state is determined that the vehicle is in substantially the straight running condition when the absolute value of at least one of the lateral acceleration, the yaw rate or the steering angle is smaller than the predetermined value.
Namely, in the four wheel drive vehicle control apparatus according to the first or sixth aspect of the present invention, the turning condition of the vehicle is determined with the turning state determining unit, the actual left and right wheel differential speed at the differential limiting unit provided on at least either one of the front wheel side or the rear wheel side is calculated according to the turning condition of the vehicle with the actual left and right wheel differential speed calculating unit. The target left and right wheel differential speed at the differential limiting unit is set with the target differential speed setting unit. Then, the differential limiting torque that can be obtained on the basis of the information at the differential limiting unit is set at 0 with the differential limiting torque calculating unit in the event that at the differential limiting unit, an inside wheel speed falls below the outside wheel speed by the preset threshold value in the turning state, whereas the differential limiting torque that can be obtained on the basis of the information at the differential limiting unit is calculated on the basis of the target left and right wheel differential speed. The actual left and right wheel differential speed with the differential limiting torque calculating unit in the event that the inside wheel speed exceeds the outside wheel speed by the preset threshold value in the turning condition. The front and rear wheel differential limiting torque setting unit sets the front and rear wheel differential limiting torque on the basis of a differential limiting torque on the basis of the information at the differential limiting unit which is calculated with the differential limiting torque calculating unit.
Here, as is described in the second or seventh aspect of the present invention, the threshold value used at the differential limiting torque calculating unit to compare the inside wheel speed with the outside wheel speed in the turning condition is, to be specific (especially), 0 or the value which results when the inside wheel speed and the outside wheel speeds are identical (or equal) to each other.
In addition, as is described in the third or eighth aspect of the present invention, in the event that the turning state determining unit determines the turning condition of the vehicle substantially as the straight running condition, the actual left and right wheel differential speed calculating unit regards the actual left and right wheel differential speed at the differential limiting unit as an absolute value of a left and right wheel differential speed. The differential limiting torque calculating unit calculates a differential limiting torque on the basis of information at the differential limiting unit on the basis of the target left and right wheel differential speed and the absolute value of the left and right wheel differential speed.
As is described in the fourth or ninth aspect of the present invention, the turning state determining unit determines the turning condition of the vehicle from at least one of the lateral acceleration, the yaw rate and the steering angle. When the absolute value thereof is smaller than a predetermined value, as is described in the fifth or tenth aspect of the present invention, the vehicle is determined as being in substantially the straight running state.
By providing the control apparatus for the four wheel drive vehicle as described heretofore, for example, when the grip of the inside wheel reaches its limit and the inside wheel begins to spin during a sporty running (an accelerator pedal is depressed while the vehicle is turning), the driving torque is shifted to the outside wheel by the mechanical differential limiting control between the left and right wheels, and a yaw moment of the front portion of vehicle toward a turning direction is generated. As this occurs, in case the differential limiting torque between the left and right wheels is sufficient, since the inside wheel speed falls below that of the outside wheel by the predetermined threshold value, the differential limiting control of the center differential is not implemented. There is caused no risk that the differential limiting torque of the center differential interferes unnecessarily. On the other hand, in the case the differential limiting torque between the left and right wheels is insufficient, the inside wheels rotates much faster than that of the turning outside wheels, while the traction performance and the cornering performance are both reduced. Since the inside wheel speed exceeds the outside wheel speed by the threshold value, this is detected to implement the differential motion limiting control of the center differential, whereby the excessive driving torque is shifted to the other driving shaft, thereby making it possible to improve the traction performance and the cornering performance. In addition, since the inside wheel speed is slower than that of the outside wheel in a normal turning state, the differential limiting control of the center differential is not implemented, whereby the tight corner braking phenomenon can be avoided.